In general, an organic light emitting phenomenon refers to a phenomenon in which electric energy is converted into light energy of an organic material. An organic electric element utilizing the organic light emitting phenomenon usually has a structure including an anode, a cathode, and an organic material layer interposed therebetween. Here, in many cases, the organic material layer may have a multilayered structure including multiple layers made of different materials in order to improve the efficiency and stability of an organic electric element, and for example, may include a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and the like.
A material used as an organic material layer in an organic electric element may be classified into a light emitting material and a charge transport material, for example, a hole injection material, a hole transport material, an electron transport material, an electron injection material, and the like according to its function.
The most problematic issues in an organic electric element are life span and efficiency, and the situation is such that this life span or efficiency issue must be solved as displays become larger and larger.
Efficiency, life span, driving voltage, and the like are correlated with each other. For example, if efficiency is increased, then driving voltage is relatively lowered, and the crystallization of an organic material due to Joule heating generated during operation is reduced as driving voltage is lowered, as a result of which life span shows a tendency to increase.
However, efficiency cannot be maximized only by simply improving the organic material layer. This is because long life span and high efficiency can be simultaneously achieved when an optimal combination of energy levels and T1 values, inherent material properties (mobility, interfacial properties, etc.), and the like among the respective organic material layers is given.
Further, in order to solve the emission problem with a hole transport layer in a recent organic electric element, an emitting-auxiliary layer is present between the hole transport layer and a light emitting layer, and it is time to develop different emitting-auxiliary layers according to respective light emitting layers (R, G, B).
In general, an electron transferred from an electron transport layer to a light emitting layer and a hole transferred from a hole transport layer to the light emitting layer are recombined to form an exciton.
However, since materials to be used in the hole transport layer must have low HOMO values, they mostly have low T1 values, and on account of this, the exciton formed in the light emitting layer is transferred into the hole transport layer, which causes charge unbalance in the light emitting layer and thus are emitted at a hole transport layer interface.
The light emission at the hole transport layer interface has a problem in that color purity and efficiency are lowered and life span is shortened. Therefore, there is an urgent need to develop an emitting-auxiliary layer which has a high T1 values and the HOMO level of which is between the HOMO energy level of a hole transport layer and the HOMO energy level of a light emitting layer.
In addition, it is required to develop a hole injection layer material that retards penetration/diffusion of metal oxides from an anode electrode (ITO) into an organic material layer, which is one cause for the shortened life span of an organic electric element, and has stability against Joule heat generated during the operation of an organic electric element, that is, a high glass transition temperature. Also, it has been reported that a low glass transition temperature of a hole transport layer material has a great effect on the life span of an organic electric element because the uniformity of a thin film surface collapses during the operation of the element. In general, deposition is a main method of forming an OLED, and thus there is an actual need to develop a material that is durable to such a deposition method, that is, a highly heat-resistant material.
In order to allow an organic electric element to fully exhibit the above-mentioned excellent features, it should be prerequisite to support a material constituting an organic material layer in the element, for example, a hole injection material, a hole transport material, a light emitting material, an electron transport material, an electron injection material, an emitting-auxiliary layer material or the like, by a stable and efficient material. However, such a stable and efficient organic material layer material for an organic electric element has not yet been fully developed. Accordingly, there is a continuous need to develop new materials for a hole transport layer or an emitting-auxiliary layer.